| Abstract |
Using the highest resolution mode of STIS (R>114,000, 1.3-2.7 km/s),
we provide new insight into the physical properties of the highly
ionized plasma probed by the "high ions" (CIV, SiIV, NV). We use a
high-quality sample of 38 sightlines probing both the Galactic disk and
halo gas. Contrary to common belief, we find that 40-50% of the
individual components of SiIV and CIV are narrow, implying cool
temperatures of T<50,000 K. Very few of these narrow components are
consistent with photoionization by OB-type stars; photoionization from
radiatively cooling hot plasma or the remains of a hot radiatively
cooling gas is a more likely scenario. The remaining 50% is consistent
with >100,000 K plasma, but the contribution from non-thermal motions
can be significant. Thus, the broad components may be hot, collisionally
ionized gas, but some fraction may also be cooler gas with significant
non-thermal components. We finally compare the properties of the
highly-ionized gas with other gas-phase tracers (e.g., AlIII, SiII,
X-rays) in order to decipher the relationship between the various phases
of the interstellar gas. |